The present invention generally relates to an atrial cardiovertor/defibrillator and method for applying cardioverting electrical energy to the atria of a human heart in need of cardioversion. The present invention is more particularly directed to an improved atrial cardiovertor/defibrillator and method wherein after each application of atrial cardioversion therapy, the atria are paced to prevent spontaneous reversion from normal sinus rhythm back to atrial fibrillation.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life-threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness as a result of reduced cardiac output.
Atrial fibrillation occurs suddenly, and at one time could and in many cases still may only be corrected by an external defibrillator discharging electrical energy to the heart through the skin of the patient. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected R wave of the heart. The treatment is very painful and, unfortunately, most often provides patients only with temporary relief lasting but a few weeks to months.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them, which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality. Two such proposed defibrillators, although represented as being implantable, were not fully automatic and required human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators required the patient to recognize the symptoms of atrial fibrillation, with one defibrillator requiring a visit to a physician to activate the defibrillator, and the other defibrillator requiring the patient to activate the defibrillator with an external magnet.
An improved atrial defibrillator and lead system which is fully automatic in operation is described in U.S. Pat. No. 5,282,837, which issued on Feb. 1, 1994, for ATRIAL DEFIBRILLATOR AND METHOD, which patent is assigned to the assignee of the present invention and incorporated herein by reference. The defibrillator described in the aforementioned referenced patent automatically detects the presence of an atrial fibrillation episode and applies cardioverting electrical energy to the atria in timed relation to a sensed R wave of the heart. Ventricular pacing is also provided by the described defibrillator.
As can be noted from the above, atrial defibrillators are known that detect for the presence of atrial fibrillation and, if atrial fibrillation is detected, apply therapy to cardiovert the atria. Such therapy may include a single defibrillation electrical shock to the atria utilizing a monophasic, a biphasic, or other waveform shape.
It has been observed however that, in some cases, when normal atrial activity is restored, it may be restored for only a short time, one minute or less, for example, at which time the heart spontaneously reverts from normal sinus rhythm (NSR) back to atrial fibrillation. There are at least three mechanisms that may explain such an occurrence.
The first is that after a cardioverting shock, there may be localized foci of atrial tissue which activate at different times thereby eliminating the possibility of a well organized activation wavefront. The second is that after a cardioverting shock, the heart may experience a bradycardia episode and have no intrinsic rhythm. Absent such intrinsic rhythm, normal areas of the atria may activate on their own, in a disorganized manner resulting in the reinitiation of atrial fibrillation. Lastly, after a cardioverting shock, the atria may experience dispersion of refractoriness so that not all of the atrial cells will be repolarized at any one time. This would render the atria more susceptible to a premature excitation which could in turn cause reinduction of atrial fibrillation. Each of the above mechanisms provides an explanation as to how reinduction of atrial fibrillation could occur.
The present invention provides a solution to the problem of reinitiated of atrial fibrillation regardless of its cause. More particularly, the present invention contemplates pacing the atria after each application of a therapy shock to force all of the atrial tissue to activate and recover together, thereby eliminating the likelihood of a premature or disorganized ectopic beat.
Post-cardioversion pacing is described in copending application Ser. No. 08/243,858, filed May 17, 1994 for IMPLANTABLE ATRIAL DEFIBRILLATOR HAVING AN INTERMITTENTLY ACTIVATED PACING MODALITY. However, the post-cardioversion pacing contemplated therein is to sustain a normal cardiac rate at a time when the normal conduction system of the heart is stunned immediately after an attempted cardioversion. Such pacing is referred to as bradycardia pacing and is performed at a relatively slow rate not necessarily intended to prevent the reinitiation of atrial fibrillation.